Oracle Questions

WATER MARK is a divided segment of used and free blocks. Blocks which are below high WATER MARK i.e. used blocks, have at least once contained some data. This data might have been deleted later. Oracle knows that blocks beyond high WATER MARK don’t have data; it only reads blocks up to the high WATER MARK during a full table scan. 

 - When two processes are waiting to update the rows of a table which are locked by another process, the situation is called a deadlock.
 - The reasons for it to happen are:
 * lack of proper row lock commands.
 * Poor design of front-end application
- It reduces the performance of the server severely.
- These locks get automatically released automatically when a commit/rollback operation is performed or any process is killed externally.

DocumentDB is a NoSQL document-oriented database and records saves in key-value pairs. It's same as the other NoSQL document-oriented such as MongoDB.

Shared pool in oracle contains cache information that collects, parses, interprets and executes SQL statements that goes against database. This shared pool acts like a buffer for these SQL statements.

Control File is used for:
- Database recovery.
- Whenever an instance of an ORACLE database begins, its control file is used to identify the database and redo log files that must be opened for database operation to go ahead.

Personal Oracle lite is single user database for distributed and mobile computing. It supports a full range of Java database features as well as advanced replication. Or in other words it is Oracle's lightweight, fully-functional, single-user object- relational database. A small, zero administration embedded database.

Some of the features supported:

- Java Support

- Java Store Procedure Support

- Java triggers Supported

- Multi-column indexing and outer joins.

- Easy to use graphical user interface

- Supports both object and relational database models

- Simultaneous viewing of classes and relational tables

A package is a collection of related PL/SQL objects. The package contains a body and a specification. The package specification has the declaration which is public and can be used in the PL/SQL sub programs inside the package.

The package body holds the implementation of all the PL/SQL objects declared in the specification.

Example of a PL/SQL Package.

CREATE OR REPLACE PACKAGE emp_data AS 

PROCEDURE add_employee (

      ename VARCHAR2,

      job VARCHAR2,

      mgr NUMBER,

      sal NUMBER,

      deptno NUMBER);

END emp_actions;

CREATE OR REPLACE PACKAGE BODY emp_data AS 

PROCEDURE add_employee (

       ename VARCHAR2,

       job VARCHAR2,

       mgr NUMBER,

       sal NUMBER,

       deptno NUMBER) IS

BEGIN

         INSERT INTO emp VALUES (empno_seq.NEXTVAL, ename, job, mgr, SYSDATE, comm, deptno);

END add_employee;

END emp_data;

Advantages of  PL/SQL packages :

Packages are easier for application designing, encapsulating data, additional functionality and better performance. An application has various modules which can be placed in packages and handled easier.

Aggregate functions in SQL are used to perform calculation on data. These functions are inbuilt in SQL and return a single value.

SUM( )

SUM function returns the sum or addition of all NOT NULL values of a column. For e.g. I have a Table employee with the fields id, name, salary and I want the sum of all salaries, I can use SUM function as shown
SELECT SUM(emp_salary) from employee;
Hence, if my column emp_salary has values 20,000, 22,000, 21,000; the output will be 63,000

AVG( )

 
AVG function returns the average of all NOT NULL values of a column. For e.g. I have a Table employee with the fields id, name, salary and I want the average of all salaries, I can use AVG function as shown
SELECT AVG(emp_salary) from employee;
Hence, if my column emp_salary has values 20,000, 22,000, 21,000; the output will be 21,000

COUNT( )

COUNT function returns the number of rows or values of a table. For e.g. I have a Table employee with the fields id, name, salary and I want the count of all rows, I can use COUNT function as shown
SELECT COUNT(*) from employee;

Max ( ) and Min ( )

MAX function returns the largest value of a column in a table. For e.g. I have a Table employee with the fields id, name, salary and I want the maximum salary of an employee, I can use MAX function as shown
SELECT MAX(emp_salary) from employee;
Hence, if my column emp_salary has values 20,000, 22,000, 21,000; the output will be 22,000

MIN function returns the smallest value of a column in a table. For e.g. I have a Table employee with the fields id, name, salary and I want the minimun salary of an employee, I can use MIN function as shown
SELECT MIN(emp_salary) from employee;
Hence, if my column emp_salary has values 20,000, 22,000, 21,000; the output will be 20,000